Etymology
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rested (adj.)
"refreshed by sleep," c. 1400, past-participle adjective from rest (v.).
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restful (adj.)

mid-14c., "characterized by or conducive to rest, characteristic of a contemplative life;" late 14c., "quiet, peaceful, tranquil;" from rest (n.1) + -ful. Related: Restfully; restfulness.

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unrest (n.)
mid-14c., from un- (1) "not" + rest (n.). Similar formation in West Frisian onrest, Middle Low German unreste, German unrast, Middle Dutch onruste.
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restless (adj.)

late 14c., restles, "finding no rest or sleep, unable to rest; uneasy in mind or spirit," from rest (n.1) + -less. A general Germanic compound (Frisian restleas, Dutch rusteloos, German rastlos, Danish rastlös). The meaning "stirring constantly, desirous of action" is attested from late 15c. Related: Restlessly; restlessness. Old English had restleas "deprived of sleep."

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restroom (n.)

also rest room, rest-room, 1887, "room set aside for rest and quiet" (in a workplace, public building, etc.); see rest (n.1) + room (n.). As these often later had (or were required to have) accessory toilet-rooms, by 1930s the word came to be a euphemism for "lavatory, toilet."

A. ... I walked into the rest room and three or four men went in there, talking, and it seemed to me as though the place was sort of disorganized.
Q. (By Mr. CARMODY.) What do you mean by "rest room"? Do you mean the toilet?
A. That is right.
Q. It really wasn't a rest room?
A. The rest room was upstairs, over the toilet.
[NLRB vs. Pennsylvania Greyhound Lines, Inc., transcript of record, U.S. Supreme Court, October Term, 1937]
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restive (adj.)

early 15c., restif, restyffe, of animals, "not moving forward," from Old French restif "motionless, brought to a standstill" (Modern French rétif), from rester "to remain" (see rest (v.2)).

Rare or archaic in the original sense; the prevailing meaning "refusing to stand still" especially of horses (attested by 1680s) probably is based on the notion of "unmanageable, impatient in restraint" in reference to a horse refusing to go forward (1650s).

But it also is perhaps influenced by rest (v.), an old aphetic form of arrest "to stop, check," and by confusion with restless. Compare resty in the same sense, 1510s of horses, c. 1600 of persons. Related: Restively; restiveness.

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re- 

word-forming element meaning "back, back from, back to the original place;" also "again, anew, once more," also conveying the notion of "undoing" or "backward," etc. (see sense evolution below), c. 1200, from Old French re- and directly from Latin re- an inseparable prefix meaning "again; back; anew, against."

Watkins (2000) describes this as a "Latin combining form conceivably from Indo-European *wret-, metathetical variant of *wert- "to turn." De Vaan says the "only acceptable etymology" for it is a 2004 explanation which reconstructs a root in PIE *ure "back."

In earliest Latin the prefix became red- before vowels and h-, a form preserved in redact, redeem, redolent, redundant, redintegrate, and, in disguise, render (v.). In some English words from French and Italian re- appears as ra- and the  following consonant is often doubled (see rally (v.1)).

The many meanings in the notion of "back" give re- its broad sense-range: "a turning back; opposition; restoration to a former state; "transition to an opposite state." From the extended senses in "again," re- becomes "repetition of an action," and in this sense it is extremely common as a formative element in English, applicable to any verb. OED writes that it is "impossible to attempt a complete record of all the forms resulting from its use," and adds that "The number of these is practically infinite ...."   

Often merely intensive, and in many of the older borrowings from French and Latin the precise sense of re- is forgotten, lost in secondary senses, or weakened beyond recognition, so that it has no apparent semantic content (receive, recommend, recover, reduce, recreate, refer, religion, remain, request, require). There seem to have been more such words in Middle English than after, e.g. recomfort (v.) "to comfort, console; encourage;" recourse (n.) "a process, way, course." Recover in Middle English also could mean "obtain, win" (happiness, a kingdom, etc.) with no notion of getting something back, also "gain the upper hand, overcome; arrive at;" also consider the legal sense of recovery as "obtain (property) by judgment or legal proceedings." 

And, due to sound changes and accent shifts, re- sometimes entirely loses its identity as a prefix (rebel, relic, remnant, restive, rest (n.2) "remainder," rally (v.1) "bring together"). In a few words it is reduced to r-, as in ransom (a doublet of redemption), rampart, etc.

It was used from Middle English in forming words from Germanic as well as Latin elements (rebuild, refill, reset, rewrite), and was used so even in Old French (regret, regard, reward, etc.).

Prefixed to a word beginning with e, re- is separated by a hyphen, as re-establish, re-estate, re-edify, etc. ; or else the second e has a dieresis over it: as, reëstablish, reëmbark, etc. The hyphen is also sometimes used to bring out emphatically the sense of repetition or iteration : as, sung and re-sung. The dieresis is not used over other vowels than e when re is prefixed : thus, reinforce, reunite, reabolish. [Century Dictionary, 1895]
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*sta- 

*stā-, Proto-Indo-European root meaning "to stand, set down, make or be firm," with derivatives meaning "place or thing that is standing."

It forms all or part of: Afghanistan; Anastasia; apostasy; apostate; armistice; arrest; assist; astatic; astatine; Baluchistan; bedstead; circumstance; consist; constable; constant; constitute; contrast; cost; desist; destination; destine; destitute; diastase; distance; distant; ecstasy; epistasis; epistemology; establish; estaminet; estate; etagere; existence; extant; Hindustan; histidine; histo-; histogram; histology; histone; hypostasis; insist; instant; instauration; institute; interstice; isostasy; isostatic; Kazakhstan; metastasis; obstacle; obstetric; obstinate; oust; Pakistan; peristyle; persist; post (n.1) "timber set upright;" press (v.2) "force into service;" presto; prostate; prostitute; resist; rest (v.2) "to be left, remain;" restitution; restive; restore; shtetl; solstice; stable (adj.) "secure against falling;" stable (n.) "building for domestic animals;" stage; stalag; stalwart; stamen; -stan; stance; stanchion; stand; standard; stanza; stapes; starboard; stare decisis; stasis; -stat; stat; state (n.1) "circumstances, conditions;" stater; static; station; statistics; stator; statue; stature; status; statute; staunch; (adj.) "strong, substantial;" stay (v.1) "come to a halt, remain in place;" stay (n.2) "strong rope which supports a ship's mast;" stead; steed; steer (n.) "male beef cattle;" steer (v.) "guide the course of a vehicle;" stem (n.) "trunk of a plant;" stern (n.) "hind part of a ship;" stet; stoa; stoic; stool; store; stound; stow; stud (n.1) "nailhead, knob;" stud (n.2) "horse kept for breeding;" stylite; subsist; substance; substitute; substitution; superstition; system; Taurus; understand.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tisthati "stands;" Avestan histaiti "to stand;" Persian -stan "country," literally "where one stands;" Greek histēmi "put, place, cause to stand; weigh," stasis "a standing still," statos "placed," stylos "pillar;" Latin sistere "stand still, stop, make stand, place, produce in court," status "manner, position, condition, attitude," stare "to stand," statio "station, post;" Lithuanian stojuos "I place myself," statau "I place;" Old Church Slavonic staja "place myself," stanu "position;" Gothic standan, Old English standan "to stand," stede "place;" Old Norse steði "anvil;" Old Irish sessam "the act of standing."

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acquiesce (v.)

1610s, "remain at rest" (a sense now obsolete); 1650s as "agree tacitly, concur," from French acquiescer "to yield or agree to; be at rest," (14c.), from Latin acquiescere/adquiescere "become quiet, remain at rest, rest, repose," thus "be satisfied with, be content," from ad "to" (see ad-) + quiescere "become quiet," from quies (genitive quietis) "rest, quiet" (from PIE root *kweie- "to rest, be quiet"). Related: Acquiesced; acquiescing.

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quiet (n.)

c. 1300, "freedom from disturbance or conflict; calm, stillness," from Old French quiete "rest, repose, tranquility" and directly from Latin quies (genitive quietis) "a lying still, rest, repose, peace" (from PIE root *kweie- "to rest, be quiet").

From late 14c. as "inactivity, rest, repose;" from c. 1400 as "absence of noise."

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